红外$ν_1+ν_3$组合模式表征CO $_2$冰薄膜
Characterization of thin film CO$_2$ ice through the infrared $ν_1+ν_3$ combination mode
摘要：
在尘埃颗粒的冰帽中有大量的二氧化碳;从大约650 cm $^{-1}$处弯曲剖面的双峰分裂中可以推断出，有些是以纯晶体形式存在的。为了研究CO $_2$如何从富含水的混合物中分离成纯净的形式以及它如何结晶，我们使用反射吸收红外光谱（RAIRS）来研究纯CO $_2$冰的结构变化作为冰厚度和温度的函数。我们发现，$^{-1}$在3708 cm处的$ν_1+ν_3$组合模吸收曲线提供了一个很好的探针来量化CO $_2$冰的结晶度。在20 ~ 30 K之间，CO $_2$分子发生了有序转变，而CO $_2$分子的扩散在更高的温度下变得明显.在ISM冰中形成纯结晶CO $_2$的过程中，速率限制过程是CO $_2$分子在冰中的扩散/分离，而不是在形成CO $_2$的簇/岛之后从非晶到结晶的相变。
英文摘要：
Carbon dioxide is abundant in ice mantles of dust grains; some is found in the pure crystalline form as inferred from the double peak splitting of the bending profile at about 650 cm$^{-1}$. To study how CO$_2$ segregates into the pure form from water-rich mixtures of ice mantles and how it then crystallizes, we used Reflection Absorption InfraRed Spectroscopy (RAIRS) to study the structural change of pure CO$_2$ ice as a function of both ice thickness and temperature. We found that the $ν_1+ν_3$ combination mode absorption profile at 3708~cm$^{-1}$ provides an excellent probe to quantify the degree of crystallinity in CO$_2$ ice. We also found that between 20 and 30~K, there is an ordering transition that we attribute to reorientation of CO$_2$ molecules, while the diffusion of CO$_2$ becomes significant at much higher temperatures. In the formation of pure crystalline CO$_2$ in ISM ices, the rate limiting process is the diffusion/segregation of CO$_2$ molecules in the ice instead of the phase transition from amorphous to crystalline after clusters/islands of CO$_2$ are formed.
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